Diffraction is a physical phenomena universally observed whenever a field of propagating energy impinges on an obstacle, causing the field to change the direction of its flow. In the case of light, for example, parallel rays reaching a mirror or lens appear to bend at the rim, making it impossible to focus the rays into a true point. A star, for example, would be imaged as a disc surrounded by rings setting the so-called "diffraction-limits" to the resolving capability of imaging instruments. This is given by EQU R=1.22 L/A (1)
for a lens or reflector of diameter A focusing radiation of wavelength L. In lasers and sonar beams alike, diffraction causes the spreading of the output field, making it lose its concentration as the distance from the source increases.
Although diffraction is thus a `natural phenomena` like gravitation, it is the object of this invention to show that diffraction effects can indeed be prevented from forming, just as gravity, by analogy, is `cancelled` in satellites, where the centrifugal force is equal to and opposite to the gravitational force, resulting in zero gravity. That diffraction-free fields are indeed possible has been shown, albeit in a limited way, by recently published theoretical and experimental results.
Toraldo di Francia invented so-called superresolving filters which use an aperture divided into annular regions, and succeed in giving a very sharp, but extremely faint central maximum. However a large amount of new diffraction is introduced in the side-lobes of the image function, making such filters of no practical value. These are described in "Reappraisal of Arrays of Concentric Annuli as Superresolving Filters", Journal of the Optical Society of America, Vol.72, pp.1287-1291 (1982). More recently J. Durnin, writing in The Journal of the Optical Society of America (A), Vol.4, No.4 April 1987, "Exact solutions for non-diffracting beams. I. The scalar theory", pp. 651-654 has shown theoretically how the wave equation can yield diffraction-free modes. Experimental demonstartion of these beams was given by J. Durnin et al in Physical Review Letters, "Diffraction-Free Beams", Apr. 13, 1987 Vol.58 No.15 pp.1499-1501. Here the aperture is limited to a single thin ring, making the resulting beam very faint. Without doubt, however, as in the super-resolving filters, special methods implemented near the aperture rim have indeed succeeded in cancelling diffraction effects. V. Tamari, writing in Optoelectronics (Mita Press, Tokyo, Japan) Vol.2, No.1, June 1987, "The Cancellation of Diffraction In Wave Fields" pp.59-81 has described a totally different method where the full area of the aperture is utilized, but with a phase change mostly near the rim, giving a bright yet diffraction-free (DD) field.
In all these studies it was shown theoretically and experimentally that diffraction effects could be cancelled by implementing various methods at the aperture-(x,y)-plane. As will be shown below, the current proposal is to introduce a phase change D(x,y) to the field either at the open aperture (such as that of a laser) or to the focusing optics of a lens or reflector, giving diffraction-unlimited focused images or diffraction-free beams. These and other objects of the invention will be made more apparent as the specification proceeds.